Attitude control in many spacecraft is implemented using one or more control moment gyros (CMGs). A CMG is a torque-generating mechanism that may include a rotating flywheel, or rotor, that is mounted in a gimbal frame and that rotates about a spin axis. The gimbal frame may be rotationally mounted about one or more gimbal axes, which are perpendicular to the spin axis. Additionally, one or more actuators may be coupled to the gimbal frame and, in response to attitude control commands, may rotate the gimbal frame about one or more of the gimbal axes. By rotating the frame about one or more of the gimbal axes at certain rates, torques can be generated in the spacecraft body to effect spacecraft attitude control.
As was noted, one of the components of a CMG is a spinning rotor. The rotor is rotationally supported using one or more bearing assemblies. The bearing assemblies are predominantly rolling element contact bearings, which are many times the limiting factor in the life of a CMG. The bearings are supplied with an initial quantity of lubricant, such as oil, to reduce rotational friction losses and to mitigate bearing wear. Depletion of this lubricant can induce failure. To address this problem, magnetic bearings have been used in some instance, but this results in increased power demand. Another way this problem has been addressed is to provide a system to periodically replenish the lubricant. Thus, an oil delivery system is provided on many CMGs.
At least one known oil delivery system includes a pressurized oil reservoir, a solenoid valve, and an oil supply line. The pressurized oil reservoir is a compressible bellows that is filled with oil. The oil within the bellows is continuously pressurized by a spring that biases the bellows toward compression. The oil supply line is in fluid communication with the CMG bearing assemblies, and the solenoid valve is in fluid communication with the reservoir and the supply line. Thus, in order to supply oil to the CMG bearing assemblies, the solenoid valve is momentarily opened, releasing a volume of oil into, and through, the supply line and to the CMG bearing assemblies. To confirm the amount of oil released, linear potentiometers have been incorporated into the system to measure the amount of bellows travel when the solenoid valve is opened.
Although the above-described oil supply system is believed to be generally safe and reliable, under certain circumstances it may exhibit certain drawbacks. For example, because the oil reservoir is continuously pressurized, if the solenoid valve leaks or remains open for an unintended length of time, an excessive amount of oil could be supplied to the CMG bearing assemblies. In a worst-case scenario, the entire contents of the oil reservoir could be inadvertently emptied to the CMG bearing assemblies. An excessive amount of oil in the CMG bearings could result in degradation of CMG performance and possible failure. As a result, the volume of the reservoir and/or oil contained within the reservoir is restricted to an amount that will not result in degradation or failure in the unlikely event the entire reservoir is emptied to the CMG bearings. This reduces the amount of oil that can be stored in the reservoir, which can adversely affect mission lifetime.
In addition to the above, the use of a solenoid valve and potentiometers can increase system costs and complexity. Moreover, the weld process that is used to join the valve in the system, and the calibration procedures associated with system implementation can be unduly complex, time-consuming, and costly.
Accordingly, it is desirable to provide a system and method for delivering lubricant to one or more CMG spin bearings that addresses one or more of the above-noted drawbacks. Namely, a lubricant delivery system and method that reduces the likelihood of delivering excessive amounts of lubricant to the spin bearings, which can increase mission lifetime by having a larger amount of deliverable oil, and that reduces system fabrication and maintenance complexity, time, and associated costs. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.